Known in the art is an exhaust purification system of an internal combustion engine which arranges an exhaust purification catalyst in an engine exhaust passage and which arranges a hydrocarbon feed valve upstream of the exhaust purification catalyst inside the engine exhaust passage, wherein the exhaust purification catalyst has the property of reducing the NOx which is contained in the exhaust gas if making the concentration of hydrocarbons which flow into the exhaust purification catalyst vibrate within a predetermined range of amplitude and within a predetermined range of period and has the property of being increased in storage amount of the NOx contained in the exhaust gas if making a vibration period of the hydrocarbon concentration longer than the predetermined range and wherein a first NOx removal method which removes NOx contained in the exhaust gas by injecting hydrocarbons from the hydrocarbon feed valve at a period within the predetermined range and a second NOx removal method which makes the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst rich to release the stored NOx from the exhaust purification catalyst when the amount of NOx stored in the exhaust purification catalyst exceeds a predetermined allowable value are selectively used (for example, see PTL 1).
In this internal combustion engine, in case where the NOx removal action by the second NOx removal method is being performed, when NOx should be released from the exhaust purification catalyst, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made rich. At this time, as a rich control for making the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst rich, a cylinder rich control which generates a rich air-fuel ratio combustion gas in the cylinder is used. Furthermore, in this internal combustion engine, when SOx should be released from the exhaust purification catalyst, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made rich. However, in this internal combustion engine, at this time, it is unclear if an exhaust rich control which feeds hydrocarbons from the hydrocarbon feed valve to make the air-fuel ratio of the exhaust gas rich is being used or the above-mentioned cylinder rich control is being used.